Neurogenesis in the fish retina

Abstract

The retinas of teleost fish have long been of interest to developmental neurobiologists for their persistent plasticity during growth, life history changes, and response to injury. Because the vertebrate retina is a highly conserved tissue, the study of persistent plasticity in teleosts has provided insights into mechanisms for postembryonic retinal neurogenesis in mammals. In addition, in the past 10 years there has been an explosion in the use of teleost fish-zebrafish (Danio rerio) in particular-to understand the mechanisms of embryonic retinal neurogenesis in a model vertebrate with genetic resources. This review summarizes the key features of teleost retinal neurogenesis that make it a productive and interesting experimental system, and focuses on the contributions to our knowledge of retinal neurogenesis that uniquely required or significantly benefited from the use of a fish model system.

FIG. 1. Histology of the vertebrate retina

(A) Diagram illustrating the cell types of the vertebrate retina. (Reproduced with permission from WebVision http://webvision.med.utah.edu/.) (B) Radial cryosection of zebrafish retina, showing retinal pigmented epithelium (rpe), outer and inner segments of photoreceptors (os/is), outer nuclear layer (onl) containing photoreceptor nuclei, outer plexiform layer (opl), inner nuclear layer (inl), inner plexiform layer (ipl), ganglion cell layer (gcl), and nerve fiber layer (nfl).

FIG. 2. Spatiotemporal patterns of neurogenesis in the fish retina

(A) Radial retinal cryosection, and (B) whole mounted eye of zebrafish embryo, depicting sequential, fan-shaped waves of cell production, with cells of the ganglion cell layer (gcl; green profiles) generated from 24–36 hpf, those of the inner nuclear layer (inl; blue profiles) generated from 36–48 hpf, and those of the outer nuclear layer (onl; red profiles) generated from 48–60 hpf (Hu and Easter, 1999). (C) Orientation of terminal mitosis predicts cell fate. Circumferential divisions (within the plane of the image) of identified progenitors lead to asymmetric fates, while radial divisions (perpendicular to the plane of the image) of identified progenitors lead to symmetric fates; i.e., two ganglion cells (gc) (Poggi et al., 2005). (Figure modified with permission from Stenkamp et al., 2002, Fig. 3J.) (D) Ectopic sites of terminal mitosis are associated with abnormal migration and ectopic differentiation (Pujic and Malicki, 2001). (E) Whole mounted zebrafish retina hybridized with a combination of cRNA probe corresponding to blue cone opsin (b; visualized in green) and UV cone opsin (uv; visualized in red) (F) Illustration of the cone mosaic of the zebrafish retina. Red profiles correspond to red-sensitive cones; green profiles to green-sensitive cones, etc. (Raymond et al., 1993).

FIG. 3. Midline hedgehog signaling is required to establish cell -intrinsic timing of retinal neurogenesis

(A) At the time of neurulation (10–15 hpf), the Hh signal from the prechordal plate (red) (Masai et al., 2000; Stenkamp and Frey, 2003a) regulates pax2 expression in optic stalk and separates the eye fields ( Macdonald et al., 1995). Evidence suggests that Hh signaling also establishes a proximal–distal gradient of an unknown intrinsic factor (Kay et al., 2005) that predicts the fan gradient (Hu and Easter, 1999; Li et al., 2000) of neurogenesis, as revealed by the spatiotemporal pattern of ath5 expression ( Kay et al., 2005; Stenkamp and Frey, 2003a). (B) If Hh signaling is blocked by cyclopamine treatment at the time of neurulation ( Kay et al., 2005; Stenkamp and Frey, 2003a), pax2 expression may be disrupted (Stenkamp and Frey, 2003a), and neurogenesis is either blocked or proceeds with inappropriate timing ( Kay et al., 2005; Stenkamp and Frey, 2003a). N, nasal; V, ventral.

FIG. 4

Retinal neurogenesis and the rod lineage in teleost fish (see also Otteson and Hitchcock, 2003). Radial cryosection of larval zebrafish retina; only dorsal retina is shown. Orange profiles represent retinal stem cells of the circumferential germinal zone (cgz) or pax6⁺ cells of the inner nuclear layer (inl) residing at the apex of the rod lineage. Yellow profiles represent NeuroD⁺ proliferative progenitor cells of the rod lineage (Hitchcock and Kakuk-Atkins, 2004); those residing in the outer nuclear layer (onl) are referred to as rod precursors (Raymond and Rivlin, 1987). Dark stripes represent rod photoreceptors; red stripes represent red cone photoreceptors (the remaining photoreceptor types are not depicted in this illustration to prevent clutter). Dark asterisk (*) shows the location of the “youngest” rod photoreceptor (closest to the cgz) with respect to the “youngest” cone photoreceptor (red asterisk) (Stenkamp et al., 1997; Wan and Stenkamp, 2000). Note the region of onl containing new cone photoreceptors that have not yet differentiated. This region has been referred to as the circumferential larval zone (Otteson and Hitchcock, 2003).

FIG. 5. Retinal regeneration: the process and putative stem cells

(A) Retinal cryosection of undamaged zebrafish retina, with superimposed depiction of probable sources of new retina following a lesion: Müller glia (green profiles) (Wu et al., 2001; Yurco and Cameron, 2005) and cells of the rod lineage (orange and yellow profiles) (Raymond et al., 1988; Wu et al., 2001). (B, C, D) The same cryosection, but images have been altered to represent (B) loss of the photoreceptor layer following a laser lesion or light damage, (C) surgical removal of a piece of retina, or (D) complete loss of neurons following treatment with ouabain. In each case, cell proliferation is increased, as depicted by the positions of the white profiles, and Müller glia reenter the cell cycle (Wu et al., 2001; Yurco and Cameron, 2005) (although this has not been documented for the ouabain lesion). For surgical lesions, proliferation also increases at sites not immediately adjacent to the lesion (Cameron, 2000). (E, F, G) Images of a cryosection have been altered to represent a retina that has (E) regenerated photoreceptors lost to light damage or a laser lesion, (F) regenerated following a surgical lesion, or (G) a retina that has regenerated following a ouabain lesion. New photoreceptors are depicted by the appropriately colored stripes, and new cells in other retinal layers are depicted by gray profiles. In each case, the regenerated cone mosaic is disorganized (Stenkamp and Cameron, 2002; Stenkamp et al., 2001; Vihtelic and Hyde, 2000). In retina regenerated following surgical or chemical lesions, cells are found ectopically, in the inner plexiform layer (Hitchcock et al., 1992; Raymond et al., 1988). Onl, outer nuclear layer; inl, inner nuclear layer; gcl, ganglion cell layer.